1. Field of the Invention
The present invention relates to a method for forming a conversion coating on metal surfaces or substrates.
2. Background of the Related Art
In general, chemical conversion coatings are formed chemically by causing the surface of the metal to be “converted” into a tightly adherent coating, where either all or part of the conversion coating consists of an oxidized form of the substrate metal. Chemical conversion coatings can provide high corrosion resistance to the substrate as well as strong bonding affinity for paint. The industrial application of paint to metals generally requires the use of a chemical conversion coating, particularly when the performance demands are high.
Although aluminum protects itself against corrosion by forming a natural oxide coating, the protection is not complete. In the presence of moisture and electrolytes, aluminum alloys, particularly aluminum alloys with a high copper content, corrode much more rapidly than pure aluminum.
In general, there are two types of processes for treating aluminum to form a beneficial conversion coating. The first is by anodic oxidation (anodization) in which the aluminum component is immersed in a chemical bath, such as a chromic or sulfuric acid bath, and an electric current is passed through the aluminum component and the chemical bath. The conversion coating formed on the surface of the aluminum component offers resistance to corrosion and a bonding surface for organic finishes.
The second type of process is by chemically producing a conversion coating, which is commonly referred to as a chemical conversion coating, by subjecting the aluminum component to a chemical solution, such as a chromic acid solution, but without using an electric current in the process. The chemical solution may be applied by immersion application, by manual application, or by spray application. The resulting conversion coating on the surface of the aluminum component offers resistance to corrosion and a bonding surface for organic finishes.
Chromate based conversion coatings have been widely used in applications where maximum corrosion protection is an issue. Immersion of aluminum or aluminum alloys in a chromate conversion coating bath results in a thick, corrosion resistant film consisting of hydrated Cr (III) and Al(III) oxides. The reaction is driven by reduction of the high valent Cr(VI) ion and oxidation of the Al metal. Some of the benefits of a chromate conversion coating include hydrophobicity and self-healing properties.
Many aluminum structural parts, as well as Cd plated, Zn plated, Zn—Ni plated, and steel parts, throughout the aircraft and aerospace industry are currently being treated using this chromic acid process technology. Chromic acid conversion films, as formed on aluminum substrates, have been shown to meet a 168-hour corrosion resistance criterion, but they primarily serve as a surface substrate for paint adhesion. Because of their relative thinness and low coating weights (40–150 milligrams/ft2), chromic acid conversion coatings do not reduce the fatigue life of the aluminum structure.
However, environmental regulations in the United States, particularly in California, and in other countries are drastically reducing the levels of hexavalent chromium compounds permitted in effluents and emissions from metal finishing processes. Accordingly, chemical conversion coating processes employing hexavalent chromium compounds need to be replaced.
Some of the most investigated non-chromate conversion coatings used in treatment of aluminum alloy-based materials are described below.
Sol-Gel technology uses polymers or metal oxides either alone or mixed to form complexes by the hydrolysis of appropriate precursor compounds. Sol-Gels can form powders or thin films that inhibit corrosion on substrates.
Fluorozirconium coating technology uses complexed transition metal salts to create a thin film on a substrate material similar to a conversion coating. Specifically, zirconium is mixed with fluorine to create fluorozirconium, which reacts with the part surface to form a coating.
Cobalt-based coatings use cobalt and molybdenum to treat substrate materials. The coatings created are low in electrical resistance and are good for corrosion resistance.
Rare Earth Metal (REM) salts may be applied by heated immersion to create protective layers on substrate materials. REMs provide corrosion resistance by producing a protective oxide film.
Potassium permanganate solutions can be used to create manganese oxide films on substrates. Manganese oxide films resulting from potassium permanganate treatment closely match the corrosion resistance of traditional chromic oxide films used in conversion coatings. Potassium permanganate coatings are very effective in protecting aluminum alloys.
Fluotitanic coatings, deposited from acid solutions with organic polymers, require few process steps, and can usually be done at ambient temperatures. Although these coatings have been widely used in a variety of applications, they have not been used in the aerospace industry.
Talc coatings, which are typically applied to aluminum substrates, are resistant to corrosion. These polycrystalline coatings are applied by precipitating aluminum-lithium compounds and other anions in an alkaline salt solution.
Anodizing is a process in which a metal surface is converted to an oxide layer, producing a tough, adherent surface layer. A thick oxide layer can be produced by immersing a part in an electrolytic solution and passing an electrical current through it, similar to electroplating. Then, by placing the part in boiling water, the film's pores can be scaled. As a result, the oxide changes from one form to another.
Despite these alternatives, there is a continuing need for a conversion coating solution that will form a stable, corrosion-resistant conversion coating on metal surfaces without containing or producing toxic chemicals. There is also a need for a conversion coating solution that provides enhanced corrosion protection on a variety of substrate materials and under a variety of conditions. Additionally, it would be desirable if the conversion coating provided a suitable surface for receiving organic coatings or paints.